feat: allow linux binaries in casks

[SMillerDev]

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That was actually easier than expected.

TODO:

• Add --zap support for Linux

[SMillerDev]

Okay, as far as I can tell from https://sorbet.org/docs/class-of this should work since NeverSudoSystemCommand is a child of SystemCommand.

I'm also fine removing the types, but that seems counterproductive.

[apainintheneck]

FYI: I think this is the last reference to OnSystem::MacOSOnly in the codebase so the definition and associated types can probably be removed too.

[Rylan12]

In order to generate the linux variations in the API, I believe you'll need to update the #each loop in to_hash_with_variations for casks. This line:

brew/Library/Homebrew/cask/cask.rb

Line 464 in 55f399c

MacOSVersion::SYMBOLS.keys.product(OnSystem::ARCH_OPTIONS).each do |os, arch|

should become something like this (copied from the formula equivalent):

brew/Library/Homebrew/formula.rb

Line 2663 in 55f399c

OnSystem::ALL_OS_ARCH_COMBINATIONS.each do |os, arch|

This may cause problems until on_linux/on_macos blocks have been added to homebrew/cask since that change would cause API generation task to load every cask as Linux, so this might be something for the future.